1. Field of the Invention
The present invention relates to a universal regenerative method of continuous gas scrubbing for a fuel cell and a device for its implementation. In particular, the present invention relates to a device for removing carbon dioxide from gas (e.g., air) that is to be used as, e.g., oxidant in a fuel cell and in particular, an alkaline fuel cell and to a method of scrubbing gas for use in a fuel cell with the device.
2. Discussion of Background Information
A fuel cell (FC) is one of the oldest electrochemical devices that generate electricity, heat and water by direct electrochemical reaction of a hydrogen-rich fuel with oxygen without any harmful emissions and therefore in an extremely environmentally friendly way. The direct generation of electricity allows FCs to be highly energy efficient. FCs have been deployed as an alternative power generation technique for the future in both mobile and stationary applications, ranging from toys to scale power stations and plants, from vehicles to mobile chargers, and from household power to battlefield power. FCs are generally classified according to the nature of the electrolyte: alkaline fuel cells (AFC), proton-exchange membrane fuel cells (PEMFCs), solid oxide fuel cells (SOFCs), phosphoric acid fuel cells (PACFs) and molten carbonate fuel cells (MCFCs), each with its own characteristic, each type requiring particular materials and fuel. However, they all comprise the same essential components, namely anode support, anode catalyst layer, electrolyte, cathode support, cathode catalyst layer, bipolar plates/interconnects and sometimes gaskets for sealing/preventing leakage of gases between anode and cathode. Each fuel cell type also has its own operational characteristics, offering advantages to particular applications. This makes fuel cells a very versatile technology.
AFCs show promise as environmentally friendly electrochemical power sources for distributed cogeneration for building, and transportation applications. The traditional AFCs operate on compressed hydrogen and oxygen and generally use a solution of potassium hydroxide in water as their electrolyte. AFCs use a liquid KOH electrolyte solution because it is the most conductive of all alkaline hydroxides, and also an effective heat transfer and water management medium. In these cells, hydroxyl ions (OH−) migrate from the cathode to the anode. The hydrogen fuel is supplied continuously to the anode compartment and an oxidant (often oxygen from air) is fed continuously to the cathode compartment. At the anode, hydrogen gas reacts with the OH− ions to produce water and release electrons. Electrons generated at the anode supply electrical power to an external circuit, then return to the cathode. There the electrons react with oxygen and water to produce more OH− ions that diffuse into the electrolyte. AFCs operate at efficiencies up to 70 percent and create little pollution. Because they produce potable water in addition to electricity, they have been a logical choice for spacecraft.
Atmospheric ambient air is often used as the oxidant in an AFC. There are several hundred ppm of carbon dioxide in ambient air. By reacting with the alkaline electrolyte (usually KOH) the carbon dioxide forms carbonates which precipitate and thereby adversely affect the performance of the AFC. Accordingly, in order to increase the lifetime and maintain the performance of an AFC, scrubbing of the air (or any other gas that may be used in the fuel cell) to remove carbon dioxide therefrom is necessary.
A method and a device for carbon dioxide (CO2) removal from air for use in an AFC are known from US 2006/0123989 A1, the entire disclosure of which is incorporated by reference herein. The device comprises a booster for a stream of air (an air supercharger), connected by means of pipes and a shutoff valve with adsorbers (scrubbers) equipped with hydrated oxides of zirconium as CO2 sorbent and connected to the air inlet of a fuel cell. The shutoff valve is made in the form of switches that provide serial connection of a gas inlet and a gas outlet of one of the adsorbers to the booster for air supply and to the air inlet of a fuel cell respectively, and the gas outlet of the other adsorber through a heater to the exhaust of a fuel cell.
However, the known device exhibits a number of shortcomings. For example, the type of sorbent used in the device (hydrated oxides of transition metals) requires air having a specified temperature (60-120° C.) and a limited humidity (not more than 85%). Further, the known device does not provide protection of the sorbent against a possible pollution by a stream (flow) of hot gas for the regeneration of sorbent that arrives from a fuel cell in a scrubber. This gas may contain small particles separated from the electrodes, drops, and aerosol substances, which adversely affect a sorbent material, reducing its active life. Additionally, the design of the scrubber (adsorber) does not allow the use of sorbent in the form of small-diameter granules since the use of granules having a diameter of less than 1.5 mm and especially cylindrical granules results in a sharp increase in the gas pressure at the gas inlet of a scrubber, resulting in an increased power consumption of the device and a need of redesigning the entire FC system. Also, the device uses external air heaters (i.e., heaters located outside the device). This results in a heat loss by the air on its way to a scrubber, the need for additional space in an FC system for accommodation the external heaters and to harmful thermal effects on other components of an FC system. Finally, the air stream switches of the 4/2 type exclusively used in the device have no compact standard analogs, and special development is required that makes the entire device more expensive and reduces its reliability.
In view of the foregoing, there is a need for an improved device for removing carbon dioxide in the gas (air) for use in a fuel cell and in particular, an alkaline fuel cell.